What I do is lift the curtain and lay the bottom of it across the edge of the tub, then I sit down there. The curtain rod has yet to come crashing down on my head, but you have to be careful not to spill water outside of the tub, because it will pool up behind you and make a big mess if you’re not careful when standing up and putting the curtain back.
Hope I explained this adequately, cause it will save you some money on not getting a chair.